FIG. 8 shows a typical known circuit of this type for D/A conversion, consisting of a so-called "R-2R ladder" type circuit or network wherein a set of resistors R1.sub.0 .about.R1.sub.N each having a resistance value of 2R equal in number to the number N plus 1 (N+1) are each connected in parallel with a terminating resistor R.sub.0 which has a resistance value 2R and is connected to ground along a series of series resistors R2.sub.1 .about.R2.sub.N having a resistance value R, and ending in an output terminal V.sub.0, the individual parallel resistors being connected by switching circuits S.sub.0 .about.S.sub.N alternately between a source of reference voltage V.sub.s and ground, such switching circuits being controlled by digital signals b.sub.0 .about.b.sub.N. As is obvious, when the digital signals b.sub.0 .about.b.sub.N of each digit become high ("On"), the reference voltage V.sub.s is applied to the corresponding parallel resistors R1.sub.0 .about.R1.sub.N. Accordingly, an analog output voltage V.sub.0 is generated at the output terminal according to the following equation (1) : EQU V.sub.0 =(V.sub.s /2).times.(b.sub.0 .multidot.2.sup.-N +b.sub.1 .multidot.2.sup.-(N-1) + . . . +b.sub.N-1 .multidot.2.sup.-1 +b.sub.N .multidot.2.sup.0) (1)
Where, b.sub.0 .about.b.sub.N are 1 or 0, either high or low. The value of V.sub.0 is lowered from the value indicated by the equation (1) when a terminating resistor or adjusting resistor is connected to the output terminal due to the resistance thereof.
Thus, the ladder type D/A converter operates as a D/A converter through the switching between ON and OFF of the parallel resistors R1.sub.0 .about.R1.sub.N in accordance with the digital signals and therefore if a circuit capable of carrying out other mathematical functions required, e.g., such as addition and subtraction before or after the conversion, such circuit must be provided in addition to the D/A converter circuit.